1. Field of the Invention
The present invention relates to a wiring conductor, a method for fabricating the same, a terminal connecting assembly, and a Pb-free solder alloy, in more particular, to a wiring conductor, a method for fabricating the same, a terminal connecting assembly, and a Pb-free solder alloy used for electronic devices.
2. Description of the Related Art
Conventionally, a plating of tin, silver, gold or nickel is provided on a wiring material, in particular, on a surface of copper or copper alloy, so as to prevent the wiring material from oxidation. For example, as shown in FIG. 7, the plating is provided on a connector pin (metal terminal) 12 of a connector (connector member) 11 and on a surface of a conductor 14 of a flexible flat cable (hereinafter, referred as “FFC”) 13, in a terminal connecting assembly (terminal connecting part) for connecting the connector 11 and FFC 13. Among the materials for plating, Sn is advantageous since the cost is low, and Sn is easily transformed by a pressure caused by fitting (contact pressure), so that a contact area increases and a contact resistance can be kept low. Therefore, the wiring material on which the Sn-plating is provided at its surface is generally and broadly employed.
For the alloy for such a Sn-plating, a Sn—Pb alloy having an excellent whisker resistance property has been conventionally used. Herein, the “whisker” is a needle-like crystal of Sn, which is generated when a stress is applied to a Sn-based material part. However, in late years, use of Pb-free materials (non-lead materials) and non-halogen materials is requested from the viewpoint for addressing the environmental concerns. Therefore, application of the Pb-free materials and non-halogen materials is demanded for various materials employed for the wiring materials.
In addition, in the field of the solder, the Sn—Pb alloy has been used conventionally, however, a Pb-free solder such as Sn—Ag—Cu-based material is now put to practical use.
The conventional arts are disclosed by Japanese Patent Laid-Open No. 11-189894, Japanese Patent Laid-Open No. 11-345737, Japanese Patent Laid-Open No. 2001-9587, Japanese Patent Laid-Open No. 2001-230151, and Japanese Patent Laid-Open No. 2002-53981.
However, in accordance with the application of Pb-free Sn-plating, a whisker which is a needle like crystal of Sn are generated from the plating particularly in Sn-plating or Sn-based alloy plating. As shown in FIG. 8, there is a risk that adjacent wiring materials (conductors 14) are short-circuited by whiskers 21.
It is assumed that the generation of the whisker can be reduced by conducting a reflow process on the Sn-plating provided by electroplating, so as to relax a stress in the Sn-plating that is believed as one of origins of the whisker.
However, a mechanism of suppressing the whisker is not understood precisely. Further, in the case where an additional external stress is applied when the Sn-plated wiring conductor is fitted into a connector, even if the reflow process is conducted, the generation of the whisker cannot be suppressed. In addition, the whisker can be suppressed by electrolysis plating of a Sn—Bi alloy, a Sn—Ag alloy or the like. However, it is reported that there is reversely more whiskers than the case of using a pure Sn-plating, when the reflow process is conducted. Since the reflow process is indispensable for mounting components in the case of manufacturing the electronic parts, there are problems in such an alloy plating. A method of providing a thin Sn-plating having a thickness of not more than 1 μm is disclosed as an effective solution for the present. However, there is a problem in that a contact resistance increases more than that of the conventional device, in particular, when being left in an environment at a high temperature (For example, see the document on the report meeting of the emergency proposal for the roadmap of JEITA lead free promotion (2005.2.17), JEITA lead-free solder practical use analysis 2005 result report (2005.6), Japanese Patent Laid-Open No. 2005-206869, Japanese Patent Laid-Open No. 2006-45665).
In addition, the generation of the whisker is concerned in accordance with the development of Pb-free in the Sn-based alloy solder.